We have recently discovered an unusual activity in mouse blood that resembles the activity mediated by the classical system of complement activation. There are several significant differences between this activity and those mediated by the known complement components, however. When EA are incubated in mouse serum dilutions or with the partially purified factor in the absence of C4 and C2, the activity goes onto the cell following first order kinetics in a calcium and magnesium ion dependent reaction. These cells can be lysed by exposure to the terminal components of complement supplied as serum-EDTA. If these cellular intermediates are instead incubated in EDTA containing buffers in the absence of serum, all activity is rapidly lost from the cell. The activity can be recovered if magnesium is resupplied to the media in the absence of any additional serum factors. The activity is ablated by 1.0 mM DFP while on the cell surface or in the fluid phase. The activity decays off of the cell in the presence of magnesium ions following first order kinetics with a half-life of 12 min at 30 C. The substance in mouse blood that mediates this function can readily be separated from C4 and C2 in a single step of ion exchange chromatography or by low ionicity precipitation. We have made a panel fo monoclonal antibodies that specifically block the function of this molecule. Two of the antibodies lead to precipitation on double diffusion and radial immunodiffusion analysis and these antibodies have been used successfully for affinity chromatography purification of the factor. We propose to define further the activities of this factor and to determine its physicochemical characteristics and composition. Its genetic control will be examined to verify preliminary experiments that indicated circulating levels were under control of H-2 complex genes. We shall determine if there is genetic polymorphism and if there are functional differences between allotypes if they are identified. Finally, it will be determined if there are analogous substances in other species.